1. Field of the Invention
The present invention relates to an optical disc drive that can write information on an optical disc.
2. Description of the Related Art
In optical disc technologies, data can be read out from a rotating optical disc by irradiating the disc with a relatively weak light beam with a constant intensity and detecting the light that has been modulated by, and reflected from, the optical disc. On a read-only optical disc, information is already stored as pre-pits that are arranged either concentrically or spirally during the manufacturing process of the optical disc. On the other hand, on a rewritable optical disc, a recording material film, from/on which data can be read and written optically, is deposited by evaporation process, for example, on the surface of a substrate on which concentric or spiral grooves are arranged. In writing data on a rewritable optical disc, data is written there by irradiating the optical disc with a pulsed light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the recording material film.
In a recordable or rewritable optical disc, when data is going to be written on its recording material film, the recording material film is irradiated with such a light beam, of which the optical power has been modulated as described above, thereby recording an amorphous mark on a crystalline recording material film. Such an amorphous recorded mark is left there by heating a portion of the recording material film that has been irradiated with a writing light beam to a temperature that is equal to or higher than its melting point and then rapidly cooling that portion. If the optical power of a light beam that irradiates the recorded mark is set to be relatively low, the temperature of the recorded mark being irradiated with the light beam does not exceed its melting point and the recorded mark will turn crystalline again after having been cooled rapidly (i.e., the recorded mark will be erased). In this manner, the recorded mark can be rewritten over and over again. However, if the optical power of the light beam for writing data (i.e., optical recording power) had an inappropriate level, then the recorded mark would have a deformed shape and sometimes it could be difficult to read the data as intended.
To read data that is stored on an optical disc or to write data on a rewritable optical disc, the light beam always needs to maintain a predetermined converging state on a target track. For that purpose, a “focus control” and a “tracking control” need to be done. The “focus control” means controlling the position of an objective lens along a normal to the surface of the optical disc (such a direction will sometimes be referred to herein as “optical disc depth direction”) so that the focal point (or at least the converging point) of the light beam is always located on the target track. On the other hand, the “tracking control” means controlling the position of the objective lens along the radius of a given optical disc (which direction will be referred to herein as a “disc radial direction”) so that the light beam spot is always located right on the target track.
In order to perform such a focus control or a tracking control, the focus error or the tracking error needs to be detected based on the light that has been reflected from the optical disc and the position of the light beam spot needs to be adjusted so as to reduce the error as much as possible. The magnitudes of the focus error and the tracking error are respectively represented by a “focus error (FE) signal” and a “tracking error (TE) signal”, both of which are generated based on the light that has been reflected from the optical disc.
Japanese Patent Applications Laid-Open Publications Nos. 9-106546 and 2010-186543 (which will be referred to herein as “Patent Documents Nos. 1 and 2”, respectively) disclose an optical disc including a read-only memory (ROM) layer on which pre-pits have already been formed and a rewritable layer on which marks will be recorded. In the optical discs disclosed in these documents, when measured from the surface of the disc (i.e., the light incident surface), those layers are located at quite different depths. That is why in reading data from the ROM layer, the light beam needs to be focused on the ROM layer. On the other hand, in reading or writing data from/on the rewritable layer, the light beam needs to be focused on that rewritable layer.
PCT International Application Publication No. 2002-039434 (which will be referred to herein as “Patent Document No. 3”) and Japanese Patent Application Laid-Open Publication No. 2003-317318 (which will be referred to herein as “Patent Document No. 4”) disclose a technique for making recorded marks on a layer of an optical disc on which pre-pits have already been formed.
Generally speaking, pre-pits are formed as integral parts of the physical structure of an optical disc when the optical disc is made. On the other hand, marks are usually recorded by irradiating an optical disc with a light beam in an optical disc drive and causing an optical structural change in the recording material film of the optical disc. That is to say, pre-pits and recorded marks are formed by totally different methods.
According to the techniques disclosed in Patent Documents Nos. 1 and 2, data should be read from the ROM layer and the rewritable layer separately, and therefore, focus and tracking controls need to be done on those two layers independently of each other. It is also necessary to generate one of two different kinds of tracking error (TE) signals depending on whether the light beam spot is currently located on the ROM layer or the rewritable layer. Since tracking guide grooves should be made on the rewritable layer, the structure of the optical disc is too complicated to make easily.
Meanwhile, according to the technique disclosed in Patent Document No. 3, a difference in light intensity in its polarization direction is detected (with attention paid to its magnetic anisotropy) as a method for separating pre-pit signals from recorded mark signals. In that case, however, an optical system for detecting the difference in the polarization direction is required, thus making the structure of the optical pickup overly complicated.
And according to the technique disclosed in Patent Document No. 4, a writing operation is performed by irradiating a portion of a reflective film on the plane where pre-pits are arranged with intense light. To read the information that has been added, a subtle variation in reflectance should be detected (see, in particular, Paragraph #0038 and FIG. 6 of Patent Document No. 4). However, since such a portion, of which the reflectance has varied slightly as a result of the writing operation, is shorter than a pre-pit, it is difficult to read the added information appropriately.
It is therefore an object of the present invention to provide an optical disc drive that can record a mark on an optical disc on which pre-pits have already been formed even if the optical disc has a simplified structure.